1. Field of the Invention
This invention relates to a hearing protector.
2. Description of the Prior Art
There are situations in military services, industry, sports involving the use of fire arms, aviation and the like, where persons are exposed to loud sounds at sound pressure levels in excess of those which are known to cause permanent hearing damage. To minimize the degree of heating damage incurred in these situations, hearing protectors in the form of earplugs, earmuffs, or a combination of earplugs and earmuffs can be worn. These hearing protectors can be classified as passive or active, and linear or nonlinear.
LINEAR-PASSIVE HEARING PROTECTORS: Linear protectors of the passive type in the form of earplugs or earmuffs are designed to attenuate incoming sound waves of different intensifies by approximately the same number of decibels before they reach the eardrum--that is, the mount of attenuation is independent of the sound pressure level of the incoming sound wave. The disadvantage of these linear hearing protectors of the passive type is that, although the wearer is protected from loud sounds, his/her ability to hear speech sounds and warning signals at sound pressure levels which normally produce low to medium loudness in the absence of the protector is impaired.
NONLINEAR-PASSIVE HEARING PROTECTOR: A nonlinear protector of the passive type may be in the form of an earplug with a small aperture. The attenuation characteristics of this type of earplugs are level-dependent and therefore nonlinear in that only incoming sound waves at levels greater than, say, 110 dB SPL are attenuated. These nonlinear earplugs therefore provide protection against intense impulse noises such as gunfire which produces peak sound pressure levels in excess of 110 dB SPL, but transmit sounds unattenuated at levels below 110 dB SPL. One disadvantage of these nonlinear earplugs of the passive type is that, at levels greater than 110 dB SPL, they provide less attenuation than the linear earplugs. Another disadvantage is that exposure to unattenuated loud sounds at levels greater than 90 dB SPL is known to cause permanent hearing damage. For a discussion of the attenuation characteristics of these nonlinear-passive hearing protectors, see for example E. A. G. Shaw, "Hearing Protector Design Concepts and Performance Limitations", Personal Hearing Protection in Industry, P. W. Alberti Editor, 1982, Raven Press, N.Y.
NONLINEAR-ACTIVE MUFF: U.S. Pat. No. 4,064,362 (Williams); U.S. Pat. No. 3,952,158 (Kyle et al.); and U.S. Pat. No. 3,394,226 (Andrews) describe non-linear protectors of the active type in the form of an muff, being designed to improve communication. A protector of this type contains, inside an earmuff enclosure, a microphone, an electronic circuit, a small loudspeaker and a battery. The microphone converts the incoming sound wave originating exteriorly of the earmuff enclosure to an electrical signal which is processed by the electronic circuit containing a nonlinear compression amplifier and other signal conditioning circuits. For incoming sound waves at sound pressure levels which normally produce low to medium loudness in the absence of the protector, the amplification characteristic of the compression amplifier is approximately linear with a fixed gain. For incoming sound waves at sound pressure levels which normally produce loud hearing sensations, the amplification characteristic of the compression amplifier is nonlinear in that its gain is inversely related to the sound pressure level of the incoming sound wave. The electrical output signal of the electronic circuit is delivered to the loudspeaker which converts the electrical signal back to sound inside the earmuff at the pinna. The electronic components of the electronic circuit arc chosen so that, for incoming sound waves which normally produce low to medium loudness, the sound pressure level at the pinna produced by the loudspeaker is approximately the same as that produced by the incoming sound wave in the absence of the earmuff. On the other hand, for incoming sound waves which normally produce loud hearing sensations, the sound pressure level at the pinna produced by the loudspeaker is less than that produced by the incoming sound wave in the absence of the earmuff as a result of the nonlinear behaviour of the compression amplifier. This protector offers unattenuated communication for speech and warning signals at low to medium sound pressure levels against a quiet background, and provides protection against harmfully loud noises by an mount approximately equal to that provided by a passive muff. The major disadvantages of this active protector include (i) difficulty in maintaining an adequate contact between the head of the wearer and the seals of the earmuff due to interference from ,the temple bars of safety glasses and/or long hair; (ii) poor condition of the seals; (iii) poor headband tension, (iv) bulkiness when used in conjunction with other safety devices such as welding shields, hard hats, helmets, respirators, or a combination of these safety devices; (v) discomfort due to the weight of the earmuff and the irritation in an environment of high ambient temperature or high humidity. Note that these disadvantages also apply to passive earmuffs.
NONLINEAR-ACTIVE CUSTOM MOULDED PLUG: Another nonlinear active protector uses the same electronic principles as the nonlinear active muff, but is in the form of a custom moulded in-the-car hearing aid with miniature hearing aid components housed in a custom-made earmould for fight fitting into the outer ear of the wearer. The miniature components include a microphone, a small loudspeaker (also known as a receiver), an electronic circuit and a battery. As in the ease of the nonlinear active earmuff, this in-the-ear protector offers unattenuated communication for speech and warning signals at low to medium sound pressure levels against a quiet background, and provides protection against harmfully loud noises. The major disadvantages of this device include: (i) difficulty in achieving a fight fit between the earmould and outer ear using existing moulding techniques for adequate sound attenuation; (ii) difficulty in maintaining the original fit due to wear, tear and shrinkage of the custom-made earmould; (iii) increased costs because the custom-made moulds must be fired by experienced technicians; (iv) clogging of the sound transmission path in the earmould by ear wax and difficulty in removing the ear wax because only the ear canal end of the sound mission path is accessible to the user;, (v) increased maintenance costs because the units must be sent back to the manufacturer or dealer for proper maintenance to remove ear wax and to repair damage caused by users who tried to clean out the wax; and (vi) lack of transferability from user to user because of the custom-made earmould. These disadvantages are commonly encountered generally in the use of in-the-ear hearing aids.